FIG. 1 is an illustration of a typical prior art orbital welding system and environment. A controller 10 provides feed speed, motor speed, and arc control signals to a welding carriage 11. Welding carriage 11 typically comprises a set of wheels 21a and 21b, at least one of which is driven by a motor (M) 16, and a welding head 17, which includes a wire feed speed mechanism (not shown) for feeding the wire or rod 20 during the welding operation. Cables 13 and 14 provide the feed speed control and motor control signals, respectively, to the welding head 17 and the motor 16, respectively. Cable 15 provides the arc welding voltage to rod 20 via welding head 17 and to the workpiece 12. In a typical environment, workpiece 12 will consist of two sections of pipe which are to be joined together. In FIG. 1, workpiece 12 represents the two sections of pipe as seen looking down the axis of the pipes. Controller 10 and welding carriage 11 operate to lay down a welding bead along the circumference of the workpiece 12 at the point where the two sections are joined together. Welding carriage 11 also comprises an opposing piece (not shown), located on the other side of workpiece 12, which provides a counterbalance for welding carriage 11 and which also serves to firmly hold the welding carriage 11 against the workpiece 12. Construction and operation of a typical controller 10 and welding carriage 11 are well known to those skilled in the art. Although the controller 10 is shown separate from the welding carriage 11 in FIG. 1, controller 10 is frequently mounted inside of the welding carriage 11.
It will be noted that, as welding carriage 11 moves around the circumference of the workpiece 12 in a clockwise direction, the welding carriage 11 will be first going uphill, then level, then downhill, then level, then uphill, etc. It is preferred that the welding seam and fill be uniform around the circumference of the workpiece 12. If the welding fill is being deposited at a fixed rate, then a faster speed will result in less fill in a given length and a slower speed will result in more fill. Therefore, the welding carriage 11 should move at a constant speed about the circumference of workpiece 12, regardless of whether welding carriage 11 is going uphill, downhill, or level. In order to achieve this a typical controller 10 will monitor the current draw of motor 16, the reverse voltage (back EMF) generated by a motor 16, or both, and provide a pulsed output voltage to motor 16 over cable 14.
FIG. 2 shows a typical output voltage pulse train provided to motor 16 by controller 10 as welding carriage 11 moves around the circumference of workpiece 12. As welding carriage 11 is moving uphill, motor 16 must provide sufficient power to overcome the inherent friction of movement of welding carriage 11 plus the effects of gravity. Therefore, controller 10 will provide pulses 30 having a long duty cycle. As welding carriage 11 moves to the top of workpiece 12 it assumes a more level orientation and motor 16 need provide only enough power to overcome the friction of movement of welding carriage 11. Therefore, controller 10 provides pulses 31 which have a smaller duty cycle. Then, as welding carriage 11 proceeds further along the circumference of workpiece 12 it will begin moving downhill and the movement of welding carriage 11 will be assisted by gravity. Therefore, to maintain a constant speed for welding carriage 11, controller 10 further reduces the pulsewidth of the pulses 32 so that the drive power of motor 16 and gravity 10 combine to keep welding carriage 11 moving at the same speed. Therefore, by sensing the back EMF and/or the current drain of motor 16, controller 10 adjusts the duty cycle, voltage amplitude, and/or the frequency of the pulses provided to motor 16 in a manner which tends to counteract the force of gravity and the friction of movement of welding carriage 11 so as to maintain a constant speed for welding carriage 11 as it travels around the circumference of workpiece 12.
Although this method of regulating the speed of welding carriage 11 does tend to promote a more uniform weld, there is still substantial room for improvement. In particular, the friction of movement of welding carriage 11 may provide insufficient braking force to maintain a constant speed going downhill even when controller 10 provides reduced pulsewidth pulses 32 or even no output pulses at all. In order to maintain better speed control some welding carriages 11 provide braking mechanisms for the wheels 21. In order to provide for sufficient braking force and provide better speed regulation, some welding carriages add a brake or clutch, which is applied at all times, so that the motor loading or unloading effect caused by going uphill or downhill, respectively, is insignificant when compared to the load provided by the brake. Of course, the motor must have sufficient power to overcome the brake loading and still move the welding carriage 11 uphill.
Also, in order to more precisely regulate the speed of welding carriage 11, some systems provide a tachometer (not shown) in conjunction with motor 16 so that controller 10 may monitor the exact speed of motor 16 and adjust the output pulsewidth accordingly. However, these techniques increase the cost, weight, and/or complexity of the controller 10/welding carriage 11 combination. Furthermore, there is generally no practical way to retrofit a braking mechanism or a tachometer into a previously existing system.
Therefore, there is a need for a speed control system for a welding carriage which will provide a uniform speed regardless of whether the welding carriage is going uphill, downhill, or level.
There is also a need for a speed control system which can be easily retrofit into an existing system.
There is also a need for a speed control system which can provide a more uniform welding speed at a low cost.
There is also a need for an automatic braking device which applies a braking force when the welding carriage rolls in a reverse direction.
There is also a need for an automatic braking device which can be easily retrofit into an existing system.